Hello Vivek
As rightly suggested by Mr Lakshmipati, TW and Anurag you need to search the internet for details. There are numerous posts in the net which cover the enterprise structure topic in terms of SD. In SCN you can find threads which will help you overcome any blockades that you might face in terms of definitions and assignments.
Anurag has rightly pointed out that you need to draw a flow chart / structure of how your organisational units such as Sales Org, Distribution channels, divisions, Sales Offices, etc are to be created in the system vis a vis the organisation for which you are going to do this configuration for.
You can use the Building Blocks provided by SAP as a base to do this activity. The Building Block applicable to Enterprise structure is 101_BB_ConfigGuide_EN_IN.
I hope this helps.
Shiv